Communication systems are used to transmit and deliver information to consumers in a variety of ways. These systems include satellite, cellular, and wireline networks, such as copper, coaxial, and fiber. The information that is passed through a communication system can be analog or digital and can be virtually anything, including telephony, video, and data.
An example of a communication system 100 is shown in FIG. 1. The communication system 100 includes headend equipment 105 for (generating forward signals that are transmitted in the downstream direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system 100 split off portions of the forward signals for provision to subscriber equipment 135, such as set-top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
For many communication systems, such as a cable distribution system, the signals that are ultimately delivered to the subscriber equipment 135 should be identical to the signals received by the headend equipment 105. In other words, the communication system 100 should not alter or otherwise distort the signals as they are being transmitted through the system 100. Various factors. however, interfere with the ability of the communication system 100 to deliver signals to the subscriber equipment 135 which are identical to the signals received at the headend equipment 105. One of these factors is the long distances over which the signals are transmitted to the multitude of subscribers. Over these long, distances, losses occur in the communication medium. hence the need for the amplifiers 125. Another factor that effects system performance is temperature. Cable distribution systems must operate over a wide rage of temperatures, such as -40.degree. C. to +60.degree. C. The performance of many components of the communication system 100, such as the amplifiers 125, is affected by temperature and fluctuations in temperature can distort the quality of the signals. In addition to distances and temperature, non-linearities in the system 100, such as within amplifiers 125, and noise can contribute to distortions that occur in downstream signals.
Because the system performance is influenced by these various factors, the system performance must be monitored and tested to ensure that quality signals are being transmitted by the headend equipment 105 and are being received at the subscriber equipment 135. For a cable distribution system, some examples of these tests or measurements include color tests to ensure that the signals produce the proper colors at the subscriber equipment 135, signal to noise ratio (S/N), carrier to noise ratio (C/N), percent modulation or depth of modulation, and hum modulation or modulation distortion of power frequencies. Many of these tests are desirable not only because they ensure proper performance of the communication system 100, but also because the Federal Communications Commission (FCC) has mandated minimum performance criteria.
The problem of hum modulation will be described in more detail with reference to FIGS. 2 and 3. Hum modulation is a measurement of a low frequency AM distortion of an RF carrier. For instance, with reference to FIG. 2, a signal S represents a signal that has been modulated with a carrier and is bounded within a signal envelope E. Hum modulation refers to an AM modulation of this envelope E which, as shown in FIG 2, causes the envelope E to fluctuate so as to have peaks and valleys. The precise measurement of hum modulation refers to the percentage of the peak to peak distortion to the peak signal level and FIG. 3 illustrates an example of the measurement of hum modulation for a single channel with a two volt horizontal sweep. According to the FCC, hum modulation shall not exceed three percent of the visual carrier level.
Hum modulation for a cable distribution system is undesirable since it distorts the picture that is available at the subscriber equipment 135. One or more horizontal bars that slowly roll through the picture at the subscriber equipment 135 is a typical manifestation of hum modulation. The AC power source and ground loops are common causes of hum modulation.
Hum modulation therefore is a significant concern in the distribution of signals in a communication system. A need exists for methods, systems, and circuits for reducing hum modulation so as to improve signal quality.